Zirconium is commonly used for forming fuel cladding, pressure tubing and other components of nuclear reactors. Zirconium is useful for such applications because of its relatively low neutron capture cross-section. The neutron capture cross-section of natural zirconium is dominated by the .sup.91 Zr isotope. The fuel economy of a nuclear reactor can be greatly improved by using .sup.91 Zr depleted zirconium in place of natural zirconium. Reduction of .sup.91 Zr in natural zirconium, containing typical alloying impurities, from its natural abundance of about 11% to 3% corresponds to a reduction in effective cross-section from 0.244 barns to 0.15 barns. Further reduction of .sup.91 Zr to 1% corresponds to a reduction in effective cross-section to 0.12 barns. The use of .sup.91 Zr depleted zirconium not only allows improved fuel efficiency, but also allows the use of thicker pressure and calandria tubes reducing tube sag and increasing safety margins. As a result, a substantial saving in the costs of retubing reactors can be realized.
Techniques exist for isotopically selective excitation and ionization of various elements. Enrichment of the uranium isotope, .sup.235 U, for nuclear power plant fuel can be achieved by atomic vapour laser isotope separation (U-AVLIS). In the U-AVLIS process, uranium metal is heated to over 2000.degree. C. to form dense atomic vapours. Dye laser beams, tuned to excite preferentially and ionize the .sup.235 U isotope, are passed through the atomic vapours. The ions, enriched in .sup.235 U are electrostatically separated from the depleted neutrals and collected. The isotopic selectivity obtained in the U-AVLIS process is very high, e.g.&gt;10.sup.4, because the spectral shifts between the .sup.235 U and .sup.238 U isotopes are much larger than the laser bandwidths. The lasers are precisely tuned to the frequency of the .sup.235 U transition to excite selectively and ionize this isotope. This approach is not practical for Zr because the isotope shifts are much smaller than the bandwidths of the lasers typically used for U-AVLIS. While lasers of sufficient resolution are available, they are characterized by very low power and hence produce unacceptably low yields for a practical .sup.91 Zr depletion process.
As a result, techniques for .sup.91 Zr depletion that do not depend on isotope shift discrimination have been proposed. U.S. Pat. No. 4,389,292, Phillips et al. issued Jun. 21, 1983 discloses a photochemical process for separating .sup.91 Zr by raising a zirconium chelate ligand from a ground state to an activated state in the presence of a scavenger which reacts with the ligand in the activated state but not in the ground state and separating out the reacted ligand. U.S. Pat. No. 4,584,073, Lahoda et al. issued Apr. 22, 1986 discloses a process for separating .sup.90 Zr by coating small bead particles with a zirconium compound such as zirconium tetrachloride and photoexciting said zirconium compound to cause a reaction of one isotope compound thereof with a scavenger gas.
Non-chemical processes for separating odd from even atomic weight isotopes using polarization selection rules are also known. In a paper entitled "Use of Angular-Momentum Selection Rules for Laser Isotope Separation", Appl. Phys. Lett. 29, 411 (1976), Balling and Wright discuss a technique for isotope-selective laser excitation of atoms which exploits the angular-momentum selection rules for the absorption of circularly polarized light. Resolved hyperfine levels are populated by stepwise excitation with two circularly polarized lasers tuned to the appropriate absorption lines. The Balling and Wright technique is stated to be effective for group III atoms and Yb. This technique requires strong hyperfine interaction and resolvable hyperfine levels. For zirconium, which has an atomic ground state of J=2, weak hyperfine interaction and many unresolvable hyperfine levels, the Bailing and Wright technique will not work efficiently. The ground state is characterized by a population of zirconium diluted over many hyperfine levels, only one of which can be accessed at a time.
In U.S. Pat. No. 4,020,350, Ducas issued Apr. 26, 1977, there is described a method for the selective excitation of odd atomic weight isotopes employing two pulsed lasers having the same handedness of circular polarization. The first laser pulse creates a coherent superposition state in an intermediate level. After the laser pulse is terminated, resonance oscillation due to hyperfine structure causes the population of the odd atomic weight isotope to be redistributed whereas the population of the even atomic weight isotope is not. According to selection rules, a second laser pulse having the same handedness of circular polarization can excite the redistributed odd atomic weight isotopes out of the intermediate state into a high lying level from which the atoms can be ionized. Although the Ducas method is described as being valid for a wide variety of more complex level structures, it is clear that such method applies only for states having relatively low J and I. This is because the Ducas method requires that the time between application of the laser pulses be set at t=.pi./.DELTA..omega. where .DELTA..omega. is the characteristic period of the frequency splitting. For zirconium which has a I=5/2 and a J=2 ground state, there exist a multiplicity (2J+1)(2I+1) of .DELTA..omega.'s which interfere in such a way that there is no single definable .omega.. The result is that the Ducas method would likely produce unacceptably low separation factors when applied to zirconium.
In a paper entitled "Effect of a Magnetic Field on the Resonant Multistep Selective Photoionization of Gadolinium Isotopes", Optics Communications, Vol. 76, No. 1, Apr. 1, 1990, Guyadec et al. disclose a multistep photoionization process for separating odd and even isotopes of gadolinium. Guyadec et al. selectively photoionizes odd isotopes (.sup.155 Gd, .sup.157 Gd), but requires the use of an autoionizing level. This level is very susceptible to external electric and magnetic fields and Helmholtz coils are required to control the magnetic field. In hostile environments typical of apparatus used to generate atomic vapours for separation, such as in an electron beam furnace, interfering electric and magnetic fields are practically difficult or impossible to control. Such fields redistribute the sublevel populations in the even isotope and destroy selectivity.
In a paper entitled "Selective ionization of Ba and Sr isotopes based on a two photon interference effect", Physical Review A, Vol. 42, No. 1, Jul. 1, 1990, Park and Diebold disclose the selective ionization of nonzero-spin atoms relative to zero-spin atoms. Park and Diebold use a one-colour two-photon resonant sequence stated to be effective for separating Ba and Sr isotopes. J=0 initial and final states are required.
In U.S. Pat. No. 5,316,635 issued May 31, 1994, there is described a method for selectively photoionizing odd mass Zr atoms in a vapour comprising even and odd mass Zr atoms. The method utilizes three resonant and one non-resonant photons to photoionize selectively the odd mass Zr atoms in a vapour comprising even and odd mass Zr atoms. The method uses quantum mechanical selection rules applicable to linearly polarized light to prepare an intermediate state which includes a magnetic sublevel in which the even mass Zr atoms are substantially unrepresented and hyperfine interactions to establish a substantially isotropic distribution of odd mass Zr atoms in the magnetic sublevels of the intermediate state. Quantum mechanical selection rules are exploited to prevent further excitation of the even isotope. The odd mass Zr atoms are excited out of the intermediate state and ionized.